Avoid crash in *Blitter::blitMask().

In MaskSuperBlitter::blitH(), iy shouldn't go negative, but it 
does, causing a crash later in the underlying blitter's 
blitMask().  For now, bail out instead of crashing.  Also add 
some SkASSERT's to try to figure out where it's going wrong.

[Stephen White, 2009-08-20 20:39:57]

Tweak whitespace and comment to match existing practice.

[Stephen White, 2009-08-20 20:47:00]

Hi Adam,

I'm adding you as reviewer since Mike seems to be away for the day, and I'm a
little anxious to get this in.  Thanks.

[reed, 2009-08-20 20:51:13]

http://codereview.appspot.com/110050/diff/1001/6
File src/core/SkScan_AntiPath.cpp (right):

http://codereview.appspot.com/110050/diff/1001/6#newcode369
Line 369: SkASSERT(ir.fTop <= (int) path.getBounds().fTop);
The cast is no good if scalar==fixed. Use this instead:

SkScalarRound(path.getBounds().fTop)

or

SkIntToScalar(ir.fTop)

Same for the other casts below

[Stephen White, 2009-08-20 21:11:51]

Fixed asserts to be SkFixed-friendly

[Stephen White, 2009-08-20 21:38:55]

http://codereview.appspot.com/110050/diff/1001/6
File src/core/SkScan_AntiPath.cpp (right):

http://codereview.appspot.com/110050/diff/1001/6#newcode369
Line 369: SkASSERT(ir.fTop <= (int) path.getBounds().fTop);
On 2009/08/20 20:51:13, reed wrote:
> The cast is no good if scalar==fixed. Use this instead:
> 
> SkScalarRound(path.getBounds().fTop)
> 
> or
> 
> SkIntToScalar(ir.fTop)
> 
> Same for the other casts below

Done.

[reed, 2009-08-21 20:45:51]

LGTM